Viscometers are widely used in the oil and other industries to measure the flow properties and especially the low temperature flow properties of various lubricants. One known instrument for measuring such properties is the Brookfield Digital Viscometer manufactured by Brookfield Engineering Laboratories, Inc. of Stoughton, Mass. 02072. This type of viscometer measures fluid viscosity by employing the principle of rotational viscometry, i.e., measurement of viscosity by sensing the torque required to rotate a spindle at constant speed while immersed in a sample fluid. The torque is proportional to the viscous drag on the immersed spindle, and thus to fluid viscosity.
FIG. 1 is an illustration of a commercial viscometer 10 which is similar to the Brookfield Digital Viscometer mentioned above. Viscometer 10 is mounted on a stand 12 with a horizontal arm 14 held by clamp 16. The clamp 16 may be vertically adjustable along an upright support 18. Viscometer 10 includes a cylindrical pivot housing 20 from which depends a shaft 22 that may support a cylindrical metallic rotor 24 or the like. The viscometer 10 rotates the rotor 24 through shaft 22 while measuring the torque or drag transmitted through the shaft by virtue of resistance encountered by the rotor immersed within a viscous fluid sample. The sample is contained within a stationary container or stator which has an inner cylindrical side wall.
To obtain a precise indication of absolute viscosity, especially for non-Newtonian fluids, it is important that the outer cylindrical wall of the rotor be equispaced from the inside cylindrical surface of the stator. With known spacing, it is possible to calculate the rate of shear of the sample fluid to obtain absolute viscosity measurements from relative viscosity. However, if spacing between rotor 24 and the stator cannot be precisely determined, i.e., if these parts cannot be centered with respect to each other, then accuracy in viscosity determination is impaired.
In the prior art viscometer disclosed supra, centering the stator and rotor is made by requiring the operator to move the stator until the operator feels that the rotor is centered within the stator. To assist in this visual alignment, viscometer 10 further includes a guard leg 32 in the form of a U-shaped member having a pair of lower straight sections 34 formed parallel to each other. These straight sections 34 extend along and are equispaced from the rotor. These straight sections 34 include upper parallel straight sections 36 connected to pivot housing 20 with screws 37. The straight sections 34, by virtue of being located between the rotor 24 and the inner cylindrical surface of the stator, assist the operator in visually centering the rotor within the stator. However, centering of these parts relative to each other by the operator still remains subjective and is dependent upon the operator's skill level. Thus, non-accurate, non-reproducible viscosity determinations are likely to occur with prior art viscometer 10.
It is accordingly one object of the present invention to provide a centering and support device which supports both the glass stator tube as well as the viscometer and rotor combination to obtain precise and consistently reproducible centering between the stator and rotor.
Another object of the invention is to provide a device providing stable support for the viscometer and rotor combination and which further provides for automatic centering of the viscometer so that the rotor is suspended perpendicular to the pivot housing.
Still another object is to provide a cylindrical support and centering device allowing the operator to calculate absolute viscosity of non-Newtonian fluids.
Yet a further object is to provide a support and centering device and method suitable for conducting viscosity measurements of low temperature fluids by maintaining an inert atmosphere above the fluid sample surface to prevent ice formation.